In paper containers suitable for long term storage of contents thereof, it is required to prevent degradation of the contents to thereby require oxygen barrier property against oxygen tending to permeate through the paper containers from the outside, and flavor barrier property for preventing dissipation of flavor components of the contents to the outside of the paper containers, so that laminated paper including an aluminum foil as a constituent member has been used. Further, there has been recently used a laminated paper having exemplarily bonded thereto a coextruded film combiningly including a deposited inorganic film, a resin having a barrier property, or the like, instead of an aluminum foil.
However, since the process for laminating these films onto a base paper is complicated and costly, it has recently started to use laminates each obtained by laminating a barrier resin onto a base paper by a coextrusion laminating method.
In case of a conventional laminate to be obtained by laminating a barrier resin onto a base paper by the coextrusion laminating method, the laminate has been typically produced by coextrusion laminating a resin layer of such a 3-kind/5-layer configuration including low-density polyethylene/adhesive resin/barrier resin/adhesive resin/low-density polyethylene, onto a base paper surface. However, it is impossible in this configuration to obtain a sufficient adhesion strength between the base paper and the low-density polyethylene, unless the temperature of the molten resin layer (hereinafter called “processing temperature” as the case may be) is set at about 315° C. or higher.
However, in case of adoption of ethylene-vinyl alcohol copolymer as the barrier resin, for example, the ethylene-vinyl alcohol copolymer tends to cause thermal decomposition at 290° C. or higher, thereby problematically and frequently causing a situation where the barrier resin is thermally degraded or gelated within an extruder such that the copolymer is unabled to be kept as a molten film to thereby cause film breakage, at such higher processing temperatures.
Further, it has been likely that the adhesive resin and low-density polyethylene also tend to cause thermal decomposition, oxidative degradation and the like, thereby possibly causing the laminate itself to become severe in odor.
It is thus conceivable to lower the processing temperature by applying an ozone treatment to that low-density polyethylene surface of the molten resin layer which is to be adhered to the base paper. However, even in this case, it is impossible to obtain a sufficient adhesion strength between the base paper and the low-density polyethylene, when the temperature of the low-density polyethylene is no higher than 290° C., at the highest.
As means for solving such a problem, there has been proposed a method for previously providing a low-density polyethylene layer on a base paper surface, and by coextrusion laminating a resin layer of a 3-kind/5-layer configuration onto it, in JP 6-55485 B, for example.
However, even in this method, there is possibly caused a defective adhesion between the low-density polyethylene layer previously provided on the base paper surface and the low-density polyethylene of the coextruded resin layer, when the processing temperature is no higher than 280° C., thereby resulting in a narrow range of processing temperature for enabling an actual processing. Moreover, this method exhibits another problem that the production process requires an excessive step so as to previously provide the low-density polyethylene layer on the base paper surface.
Meanwhile, there have been proposed a producing method of a laminate by a multi-layer coextrusion laminating method.
For example, disclosed in JP 7-16993 A is a method for coextrusion laminating a resin layer of 3-kind/3-layer comprising ethylene-vinyl alcohol copolymer/adhesive resin/polyolefin onto a base paper, such that the ethylene-vinyl alcohol copolymer side is faced to the base paper side.
Although this method enables production of a laminate at certainly lower processing temperatures, it is problematic that the extruded ethylene-vinyl alcohol copolymer in a molten state is directly contacted with air until the former is contacted with the base paper, thereby causing oxidative degradation. Further, bumps on the base paper surface are directly transferred to the ethylene-vinyl alcohol copolymer layer, such that the ethylene-vinyl alcohol copolymer layer is brought to have a non-uniform thickness when the layer is thin at about 10 μm or less, thereby possibly resulting in a deteriorated barrier property.